not only parsley,can be celery ,but just to be a bitter herbs,as they dip vegetable into a salt water (which actually represent tears)mirrors the pain felt by the Jewish slaves in Egypt
The seder plate itself isn't a symbol, it holds the symbolic items for the seder though.
Yes!
The roasted egg on the Seder plate represents one of the two sacrifices eaten on the Seder night when the Temple was standing.
It is a meal at which special symbolic foods are eaten and the Exodus is retold at length.